Audible fire alarm and method of making the same

ABSTRACT

An audible fire alarm and method of making the same. The apparatus is operative to provide one or more sharp reports signaling exposure of the alarm to a temperature exceeding a predetermined value for a time period sufficient for the alarm to attain such temperature value. It includes a casing defining a chamber within which is located a packet comprising a heatignitable explosive charge and a wrapper folded tightly thereabout to encapsulate the charge therein. Both the casing and wrapper are formed from a material having relatively good heat conductivity, such as aluminum, so that ambient temperatures to which the alarm and casing thereof are exposed are transmitted rapidly to the explosive charge to effect ignition thereof. The method includes the steps of bending a casing blank into a chamber-defining casing, of folding a measured explosive charge into an inner wrapper element, encapsulating the folded chargeequipped inner wrapper element into an outer wrapper element, of providing a substantially moisture impervious coating about the packet defined by the wrapper elements and charge therein, and of assemblying the packet with the casing.

United States Patent 91 Peretz AUDIBLE FIRE ALARM AND METHOD OF MAKING THE SAME [76] Inventor: Bernard G. Peretz, 3938 Robley Ter., Oakland, Calif. 94611 221 Filed: Apr. 19, 1971 [21] Appl. No.: 135,279

[52] US. Cl 116/105, 29/169.5, 102/37.4 [51] Int. Cl. G08b 17/00, G08b 17/08 [58] Field of Search 116/106, 104, 105, 116/11, 15, 78, 87; 29/1695; 102/37.4

[56] References Cited UNITED STATES PATENTS 138,467 5/1873 Barnes 116/105 788,225 4/1905 Sweet 116/105 1,746,426 2/1930 Hotchiss 116/105 1,840,894 1/1932 Gilbert et al.. 116/105 3,087,456 4/1963 Wood 116/105 3,299,850 l/l967 Cartier... 116/105 3,429,293 2/1969 Willat 116/105 2,533,835 12/1950 Palmer 116/102 Primary Examiner-Louis J. Capozi Att0rney.loseph B. Gardner 5 7 ABSTRACT An audible fire alarm and method of making the same. The apparatus is operative to provide one or more sharp reports signaling exposure of the alarm to a temperature exceeding a predetermined value for a time period sufficient for the alarm to attain such temperature value. It includes a casing defining a chamber within which is located a packet comprising a heatignitable explosive charge and a wrapper folded tightly thereabout to encapsulate the charge therein. Both the casing and wrapper are formed from a material having relatively good heat conductivity, such as aluminum, so that ambient temperatures to which the alarm and casing thereof are exposed are transmitted rapidly to the explosive charge to effect ignition thereof. The method includes the steps of bending a casingblank into a chamber-defining casing, of folding a measured explosive charge into an inner wrapper element, encapsulating the folded charge-equipped inner wrapper element into an outer wrapper element, of providing a substantially moisture impervious coating about the packet defined by the wrapper elements and charge therein, and of assemblying the packet with the casing.

2 Claims, 12 Drawing Figures "MFNTEUUEC 41.915 3,776,175

SHEET 2 OF 2 //v VEA/TOR. Fig, if BERNA RD 6. PERETZ AUDIBLE FIRE ALARM AND METHOD OF MAKING THE SAME This invention relates to signal devices and to a method of making the same, and it relates more particularly to an audible fire alarm operative to signal the presence of a temperature in excess of a predetermined value and to a method of fabricating such alarm.

The unexpected occurrence of fire in residential, commercial and industrial structures is an everpresent problem, and because of the significance of this problem considerable attention has been directed to alarm systems that give an early warning to the occurrence'of. fire so that those alerted by the alarmcan escape from the structure and perhaps take stepsto extinguish the fire or call for professional help to extinguish or control the same. So far as is known, the systems proposedand used for fire warning have many disadvantages and limitations among which include expense, the cost and difficulty of installation, generally unreliability, and inoperativeness under certain conditions such as the loss or failure of electric power used in such systems. As a consequence, very few residential buildings are equipped with fire alarm or detection systems of any type, and the same can be said of many industrial andcommercial buildings except where building and safety ordi+ nances and statutes require the same.

In view of the foregoing, a generally stated object of the present invention is to provide an improved fire alarm and method of making the same which overcome limitations. Further objects, among others, of the present invention are in the provision of an audible alarm or warning device for signaling the presence of a temperature in excess of a predetermined value, and which alarm is a self-contained entity that requires no electrical energy input thereto whether from AC power lines present in substantially all buildings or from-a storage battery source, thereby obviating unreliability usually associated with the interruption, distruction, deterioration, or other failures that occasionally characterizes power sources; that does not employ relay or other contacts that can become corroded or otherwise made inoperative by collection thereon of dust, dirt, and similar materials; that is very small and unobtrusive and easily mounted at any appropriate location and requires no external connections thereto; and that is very inexpensive, thereby enabling the units to be merchandised in groups or packages, and although inexpensive is nevertheless reliable without maintenance or other.

attention over long periods through which it experiences essentially no deterioration or change,

Still further objects, among others, of the present invention are that of providing an audible alarm for signaling the presence of a temperature in excess of a predetermined value, as previously explained, and which alarm includes a heat-ignitable explosive charge confined within a casing therefor; the explosive charge;

when ignited being effective to provide a loud, sharp. report and the casing being effective to deform under the force of such explosion to absorb substantially all of the energy thereof so that there is no fragmentation of the device or any portion thereof; is quick to sense the presence of ambient heat thereabout and to transmit such heat to the explosive charge very rapidly, the casing of the device being formed of a meterial having relatively good thermal conductivity and the explosive charge being encapsulated within a wrapper which also has relatively good thermal conductivity and is in heat exchange relation with the casing; and in which the explosive charge is encapsulated within a wrapper to form a packet that, together with a substantially moisture impervious coating or covering about the wrapper, isolates and protects the charge from the ambient environment that otherwise might have deleterious effects thereon.

Additional objects and advantages of the invention, especially as concems' particular features and characteristics thereof and of the method of making the alarmdevices, will become apparent as the specification continues.

Embodiments of the. invention are illustrated in the accompanying'drawings, in which:

FIG. 1: is a perspective view of a singleunit alarm= shown in association with a portion of a vertical room wall;

FIG. 2is aperspective view of a multiple-unit alarm shown in association with a portion of a vertical room wall;

FIG. 3 is adiagrammatic view depicting the step of mixing certain chemicals to provide an explosive charge;

FIGS; 4, 5, and 6 are respectively perspective views illustrating successive steps in making a heat-ignitable explosive packet for use in the alarms illustrated in FIGS. 1' and 2;

FIG. 7is aperspective view showing the packet after the steps shown in FIGS. 4, 5, and 6 have been completed;

FIGS. 8, 9, and l0;are respectively perspective views illustrating subsequent steps in the fabrication of a completedexplosive packet;

FIG. 11 is an exploded perspective view of the various elements comprising the alarm shown in FIG. 1 and-f depicting their. final assembly; and

FIG. 12 is a side view in elevation of the alarm shown in FIG. 1-, and it indicates the use of filler material to:

confine the packet within the chamber of the casing therefor.

or one-unit alarm operative to provide a single audible.

signal when heated .to a temperature in excess of some predetermined value. Multiple-unit alarms are readily provided, however, and an alarm assemblycomprising three. individual units is illustrated in FIG/2 and denoted in its entirety with the numeral 16. A multipleunit alarm is operative when heated to the ignition temperature to provide -a plurality of successive audible signals which usually are very close together i time with the. result-that a staccato sound or noise alerts one of' the presence of the ignition temperature which, in the usual case, signals the occurrence of a fire. As will be: made-more evident hereinafter, the multiple-unit alarm 16 may be formed of a plurality of single-unit alarms l5, and the time differences'in the ignition of the several alarm units is due to anomalies and vagaries,physi,- cal or otherwise, in the construction of substantially identical units.

The alarm 15 includes a casing 17 (see FIG. 11 inparticular), a packet or package 18, enclosure band 19,

sorbing, and it is also a good thermal conductor so as to transmit heat therethrough to the packet 18 confined within the chamber 21. A specific example of a meterial having the desired characteristics is aluminum, and although the gauge of the aluminum may vary considerably a thickness of approximately onesixteenth of an inch has been found quite satisfactory.

In any event, the casing 17 should be self-sustaining so that it will maintain the configuration enforced thereon in its fabrication, and it is also intended that the casing have sufficient strength to yieldably deform upon ignition of the explosive charge comprising a part of the packet 18. In this respect, the blank from which the casing 17 is made is folded downwardly and then upwardly in a somewhat U-shaped configuration to provide spaced apart, substantially parallel wall members 22 and 24 establishing the transverse limits of the chamber 21. The walls 22 and 24 are joined integrally at their lower ends by the base wall 25 of the U-shaped configuration, and at its upper end the wall 24 is turned laterally to define a flange 26 that underlies the top wall 27 of the casing and establishes the upper limit of the chamber 21. It may be noted that the flange 26 is not rivited or otherwise fixedly secured to the top wall 27 because it is intended that the walls 22 and 24 be displaced relative to each other about the hinge formed by the base wall 25 upon ignition of the explosive charge forming a part of the packet 18.

The flap 26 and top wall 27 are disposed in contiguous juxtaposition and are oriented at generally right angles with respect to the side walls 22 and 24. Since the casing 17 is integral throughout, the top wall 27 merges into the wall 22 at one end, and at its opposite end it is turned upwardly to form a hanger or mounting wall portion 28 that may be provided with an opening or aperture 29 therein to permit the alarm to be attached conveniently to a room wall 30, as shown in FIG. 1, as by means of a screw or comparable fastener 31. Accordingly, the chamber 21 and packet 18 mounted therein are offset or spaced from the plane of the room wall 30, thereby exposing both side wall portions 22 and 24 of the casing to the environmental temperature of that portion of the room adjacent the alarm 15. Thus, the room wall 30 does not tend to insolate or shield the compartment 21 and packet 18 from the environmental temperature which enables the explosive charge within the packet 18 to be elevated rapidly to its ignition temperature in the presence of environmental heat of such magnitude.

Further in this respect, the top wall 27, when the alarm 15 is mounted in the fashion illustrated in FIG. 1, is disposed in a generally horizontal orientation so that it defines a large surface area not only exposed to the environmental room temperature but also tending to stop or interrupt the upward movement of heated air and other gases from a fire, thereby causing turbulence along surface portions of the top wall 27 and adjacent side wall 24 which enhances the desired heat-exchange relationship with the casing. Therefore, the rapidity with which the casing 17 transmits environmental temperature elevations to the packet 18 is significantly augmented. Generally, then, the construction of the casing 17, especially because of its excellent heat conductivity, results in exeedingly fast response of the alarm 15 to the presence of temperature elevations within its general vicinity. Although, as heretofore stated, aluminum is an excellent material for fabrication of the casing 17, other materials having the desired characteristics can be used (copper, for example) with the gauge or thickness of the material selected to provide the requisite self-sustaining strength and deformability.

Multiple-unit alarms 16 may be comprised of a plurality of single-unit alarms l5 interconnected one with another in any configuration appropriate for a particular installation. However, one or more slightly varient forms of the alarm 15 may be employed in any particular multiple-unit alarm assembly, and the assembly 16 shown in FIG. 2 includes one alarm 15 and a somewhat modified form including two separate explosive charges. All of the casings are secured together and to a hanger which is then attached to a room wall in essentially the same manner as that described hereinbefore. In this latter reference, the room wall depicted in FIG. 2 is denoted with the numeral 34, and the hanger attached thereto by a screw-type fastener 35 is identified with the numeral 36. The hanger 36 is generally L- shaped and it has an opening therethrough (not shown) which mates with the opening 29 in the alarm 15, thereby permitting a screw fastener 37 to pass therethrough and fixedly secure the hanger to the alarm unit 15. Any suitable means may be used to interconnect the unit 15 and hanger 36 other than the screw fastener 37 such as rivets, adhesives, welding, etc., although if heat or impact are employed in the fastening arrangement used it will be precautionary to interconnect the same before the explosive packets are positioned within the various chambers provided therefor.

In addition to the alarm unit 15, the alarm 16 further includes a dual-alarm 38 having an elongated casing 39 that overlies the juxtaposed portions of the hanger 36 and easing of the alarm unit 15 and is fixedly secured thereto by the screw fastener 37. The opposite end portions of the casing 39 are folded upon themselves as in the manner of the casing 17 so as to provide a pair of chambers respectively receiving charge-equipped packets therein. Since the structural composition of such end portions of the alarm unit 38 are substantially the same as that of the alarm unit 15, the same numerals are used to identify the respectively corresponding components except that the suffixes a and b are used wherever appropriate to identify respectively corresponding elements. correspondingly, no further description of the unit 38 will be set forth because it would not be contributive to an understanding of the present invention.

The packet 18 includes a wrapper comprising inner and outer wrapper elements 40 and 41, respectively illustrated in FIGS. 4 through 7 and 8 through 10, and a charge 42 of a heat-ignitable explosive material incapsulated within the wrapper 40 and 41 and therefore forming a component of the previously-mentioned packet 18. The wrappers 40 and 41 are formed of a material having good thermal conductivity, aluminum being a specific example, and it may constitute one or more layers of a thin aluminum foil, such as the type used in considerable prevalence for household kitchen use. In the specific embodiment of the invention being considered, a four-ply aluminum foil having a width of approximately 1% inches and a folded length (as shown in FIG. 4) of approximately 3 inches, and having a flat or unquilted surface is used. The exact length and width and gauge of the wrappers 40 and 41, which can be identical, may vary considerably.

The wrappers 40 and 41 are flexible and are deformed when folded so as to maintain their folded shapes, and the charge 42 is advantageously placed in proximity to the multiple-folded end of the wrapper 40 since folding the same away from such end will result in the neatest package. The explosive charge 42 constitutes a substance that when heated to a predetermined temperature ignites and, when properly enclosed within the wrapper to form part of the packet 18, undergoes a very rapid decomposition toeffect an explosive force. In the embodiment of the invention illustrated and described herein the substance forming the explosive charge is a chemical constituting an admixture of a plurality of chemical ingredients. In particular terms, an effective substance includes flowers of sulfur and sodium bromate well mixed in the proportions of one part to two parts by volume. In terms of the specific embodiment heretofore mentioned, each part may be of the general order of i0 c lbic rnilli meters, although the precise amount of charge used in each packet 18 may be varied considerably with particular reference to the intensity of the sound or report desired to signal ignition of the charge. In this latter respect, a charge comprising the ingredients noted as the approximate quantities set forth produces as a prt of an alarm a clap or sharp piercing report of the order of 95 decibels at a distance of about 5 feet.

The strip 19 serves for the most part in the nature of a seal to discourage tampering with the alarm, and in furtherance of this object it is advantageously red in color. Any suitable material may be used for the band, and an example of a material found satisfactory is a red-colored pressure sensitive tape sold by the 3-M Company of Saint Paul, Minnesota as a Plastic Stretch Tape. The packing materials 20 and 20' also serve to prevent tampering with the alarm device and especially the packet 18 which is completely concealed by the filler material. Any convenient material may be used to fill the space within the chamber 21 not occupied by the packet 18, and desirably the material used is relatively moisture impervious. Many soft, workable semifluids are available which can be used to pack the chamber 21 after the packet 18 is located therein which then harden to provide a firm, resistive end covering for the packet. An example of one material which has been used with satisfactory success is one of the wood fillers known as Plastic Wood. Such materials often comprise fine wooden particles or sawdust together with a plastic binder and a volatile plasticizer that evaporates when the material is exposed to air, thereby permitting the substance to harden.

In fabricating the packets 18, the steps may be followed that are illustrated in FIGS. 3 through 10 to a considerable extent in diagrammatic form. Thus, FIG. 3 depicts the step of mixing two parts of sodium bromate with one part of flowers of sulfur, and these volumetric proportions are indicated by the quantities 44 and 45 located within a suitable container or vessel 46. The two ingredients are thoroughly mixed so as to completely intermingle the same, and only that quantity of material should be mixed at any time which will be used immediately. In this respect, the material should be kept dry so that ambient moisture has no deleterious effects thereon. The mixed materials are flammable and burn slowly without explosive violence if ignited in an unconfined space in the presence of ambient air.

After the ingredients have been mixed, a preselected quantity thereof is measured onto the inner wrapper 40 and, as previously explained, a volume of the general order of cubic millimeters has found to be adequate for ordinary purposes although in certain instances a charge approximating 200 cubic millimeters has been used. As shown in FIG. 4, the charge 42 is isolated or separated from direct contact with the metal foil wrapper 40 by a layer 47 of a more chemically inert material than the aluminum foil used for, the wrapper so as to prevent any chemical interaction between the charge 42 and the wrapper.-A strip of ordinary pressure-sensitive cellophane tape may be used and one having a width of about three-fourths of an inch as proven to be adequate. After the charge 42 is placed upon the isolation strip 47, the charge is pushed together along each edge into a relatively compact pile which is then confined in place by a plurality of pressure sensitivecellophane tapestrips 48, 49, and 50.

The strip 48 is longitudinally oriented and tends to.

cover most of the charge 42, whereas the strips 49 and 50 are transversely oriented and overlap each other along the contiguous edge portions thereof. The strips 48, 49, and 50 are each about three-fourths of aninch in width, but it will be appreciated that wider or narrower strips may be used, thereby resulting in the necessity for lessor or more strips as the case may be. In any case, the strips 47 through 50 tightly confine the charge 42 at a predetermined location along the wrapper 40 and physically isolate or separate the charge from contact therewith. Next the inner wrapper 40 is folded, as shown in FIG 6, to completely enclose the charge 42 and strips 47 through 50 within the interior of the wrapper 40. The wrapper may be folded progressively from one end to the other thereof, and after completely folding the wrapper in this manner, the longitudinal edge portions of the wrapper bordering the charge 42 are folded inwardly one upon another to form the small partiallycompleted packet shown in FIG. 7. The partially formed packet may then be protected by one or more pressure-sensitive tape strips'Sl and 51a which are secured aboutthe wrapper 40, the process ofv which is partially shown in FIG. 7. The wrapper 40 is tightly formed about the charge 42 so as to pack the same firmly and tightly within a confined space.

Thereafter, the partially formed packet is placed on the outer wrapper 41 and is oriented so that what had been the longitudinal axis of the inner wrapper 40 is transversely disposed with respect to the longitudinal axis of the outer wrapper 41, as illustrated in FIG. 8. The outer wrapper 41 is then progressively folded from one end to the other thereof in a manner corresponding to that of the inner wrapper 40, and the longitudinaledges of the outer wrapper are then folded inwardly one upon another to form the almost-completed packet illustrated in FIG. 9. The packet may then be rolled, as by means of a hand roller 52, to compress the packet and further compact the charge 42 therewithin.

Thereafter, the almost-completed packet is protected with a substantially moisture impervious covering which may be of any suitable type placed on the wrapper in any conventional and convenient manner. In the particular embodiment of the invention being considered, the partially-formed packet is dipped into a body of liquid 54 provided within a container- 55 to permit the liquid to cover and permeate, to the extent permissible, the interstices of the outer wrapper 41. The packet is then removed to permit the liquid to harden. Various materials can be employed to provide the desired coating, and a specific example thereof is conventional clear lacquer surface coverings which constitute a fast-drying water-impervious covering for the packet After the covering or coating about the packets 18 has dried, the packet is placed within the compartment 21 of the casing 17 (or other casing) provided for this purpose, the end portions of the chamber 21 are then packed tightly with the filler materials 20 and 20, and the tape 19 is then drawn tightly about the walls 22 and 24, as shown best in FIG. 1. After the filler materials 20 and 20' have hardened, the completed alarm may be packaged for shipment and sale to the ultimate user who then locates the unit at strategic positions so that the earliest warning will be given to the outbreak of any fire.

The completed packet 18 is quite small, and a typical example thereof has a length slightly under threefourths of an inch, a width of about one-half inch, and a thickness approximating one-eighth of an inch. Correspondingly, the casing 17 may have a width (i.e., along the length of the hinge wall 25) of approximately seven-eighths of an inch, a height from top to bottom of the chamber 21 (i.e., from the hinge wall 25 to the flap 26) of about three-fourths of an inch, and a width between the walls 22 and 24 of about three-sixteenths of an inch so that the packet 18 is snugly received therebetween. Thus, the temperature sensed by the heat-conductive casing 17 is quickly transmitted to the explosive charge 42 through the heat-conductive wrappers 40 and 41. Thus, the entire alarm unit 15 is quite small and, therefore, very unobtrusive when placed within a room to protect the same.

Embodiments of the invention formed in the manner described and having the structural characteristics noted detonate or ignite when subjected to a temperature of the general order of 350F for a time period of about 5 seconds. It will be appreciated that ignition of the explosive charge 42 is a function of time and temperature with the two factors being inversely related so that as the ambient temperature increases, the exposure period required for detonation decreases, and vice versa, the minimum temperature required for ignition of the charge 42 always being a prerequisite. The particular time and temperature relationship noted is believed to be optimumly ideal in that the ignition temperature is sufficiently high that the alarms are not detonated by the usual ambient temperatures to which they are subjected including summer attic and winter heating temperatures of commercial and residential dwellings, the summer temperatures of uncovered shipping platforms subjected to direct sunlight, etc., yet a temperature of about 350F is reached almost immediately in the presence of even the smallest fire (the tip of a match, for example), and ignition of the charge 42 then occurs within the very short period of 5 seconds.

In the event that a multiple-unit alarm 16 is employed, the various explosive charges 42 comprising components thereof are ignited in rapid succession one after another but in very close proximity to each other. Therefore, a sequence of loud, rapid reports signals the presence of an abnormally high temperature, and if one should not hear one of the reports he should hear one or more of the others comprising the overall sequence.

It has been observed through extensive testing that the various charges of multiple-unit alarms are not ignited concurrently so that no particular effort may be made to vary the quantities of the charges 42 used to form any one multiple-unit alarm. Evidently the slight differences that occur in measurement of the charges, in the degree of compaction thereof within the wrappers 40 and 41, physical or mechanical differences in positioning packets 18 within the respective chambers 21 therefor, and in the precise locations of the charges rel' ative to the support structure or casings therefor cause the slight time differences in the ignitions of the charges.

As has been indicated hereinbefore, the alarms are very reliable since they are wholly independent of power failures, are not influenced by the presence of dirt and other foreign matter, and they are almost completely protected from ambient moisture (whether airborne or otherwise) and other deleterious materials. Accordingly, the alarm devices reliably signal the presence of an abnormally high temperature that usually indicates the occurrence of a fire or development thereof. At the same time, however, the alarms are very inexpensive, small and unobtrusive, and can be made to blend with various decors preferably by anodizing the casings therefor.

It has been found that the charge 42 burns slowly without explosive force when open to ambient air, such as shown in FIG. 4. Accordingly, confinement of the charge within a wrapper, as previously described, is a requisite to proper function of the alarm. It may also be noted that upon ignition of the charge 42, the walls 22 and 24 of the relatively soft and malleable aluminum open under the force of the explosion somewhat in the manner of a flower and absorb and alternate the force of the explosion with no fragmentation.

The packet 18 fits snuggly within the chamber 21 intermediate the walls 22 and 24 essentially in contact therewith to facilitate heat transmission therebetween. Occasionally, it is advantageous to connect two of the units 15 or units 16 (or other combination) in backtoback relation and suspend the same from the ceiling of a room, and in this event the units can be taped together, if desired, and a metal wire extended through the openings 29 to maintain the units in their suspended state.

While in the foregoing specification embodiments of the invention both in terms of apparatus and method have been set forth in considerable detail for purposes of making a complete disclosure thereof, it will be apparent to those skilled in the art that numerous changes may be made in such details without departing from the spirit and principles of the invention.

What is claimed is:

l. A fire alarm, comprising a one-piece generally horizontally extending bracket having at the rear thereof a vertically extending hanger for mounting such alarm on a support wall or the like, a casing depending from the front of said bracket having opposed vertical parallel front and rear walls all in relatively remote spaced relation to the hanger whereby both of the walls of the casing will be fully exposed to the environmental temperature surrounding the casing, the sides of the casing between corresponding vertical edges of said parallel walls being also spaced apart, a packet tightly and entirely fitting within said casing containing a heatignitable explosive charge and having a moisture imbent portions of the bracket, the portion of the bracket between and spacing said casing from the hanger extending longitudinally and with its greater width horizontally so as to provide maximum responsive heat transfer to said explosive charge, and a sealing strip of narrower depth than and extending completely around and embracing the front and rear walls of the casing including the side end areas therebetween. 

1. A fire alarm, comprising a one-piece generally horizontally extending bracket having at the rear thereof a vertically extending hanger for mounting such alarm on a support wall or the like, a casing depending from the front of said bracket having opposed vertical parallel front and rear walls all in relatively remote spaced relation to the hanger whereby both of the walls of the casing will be fully exposed to the environmental temperature surrounding the casing, the sides of the casing between corresponding vertical edges of said parallel walls being also spaced apart, a packet tightly and entirely fitting within said casing containing a heat-ignitable explosive charge and having a moisture impervious wrapper encapsulating and sealing said chrage, a filling material closing the spaces between the opposed side edges of said walls, the material of which said bracket is formed being relatively soft and flexible whereby on explosion of the charge in the casing the walls will move apart without substantial fragmentation.
 2. The fire alarm of claim 1 wherein the entire bracket including said casing and hanger is formed as bent portions of the bracket, the portion of the bracket between and spacing said casing from the hanger extending longitudinally and with its greater width horizontally so as to provide maximum responsive heat transfer to said explosive charge, and a sealing strip of narrower depth than and extending completely around and embracing the front and rear walls of the casing including the side end areas therebetween. 